Now I've started with the charts I just can't stop, Time to bother you with another one.
One word of warning: I calculated the SRM and IBU values by slotting the recipes into brewing software. They are just a general indication rather than observed values.
With the colour, the trend is obvious. There's a slow and steady darkening of of the beer. Which tallies with drinkers' observations that their lovely brown beer had been transformed into black muck.
It's harder to pick trends in the bitterness, but there's a reason for that. From 1860 on, it's the Export Stout because Whitbread stopped brewing standard Single Stout. Which is why the bitterness suddenly jumps up. After that, the trend is clear: downwards.